The Drive Toward UHDI and Substrates

Estimated reading time: 3 minutes

Panasonic’s Darren Hitchcock spoke with the I-Connect007 Editorial Team on the complexities of moving toward ultra HDI manufacturing. As we learn in this conversation, the number of shifting constraints relative to traditional PCB fabrication is quite large and can sometimes conflict with each other.

Barry Matties: Darren, I recently had watched your presentation about the demands on material—resin, weave, surface, etc.—to meet the demands of higher density, finer features. What was your message to your audience regarding the state of the industry for materials in HDI and UHDI?

Darren Hitchcock: In that presentation¹, I covered some of the changes like glass style and the composition of the glass. For the longest time, we've had E-glass, which is, of course, the cost-effective option. But as electrical performance requirements have increased, we've seen more demand for low-Dk and ultra-low Dk glass. The increased trends toward that glass mean better availability and cost; as the electrical performance benefits are also decision factors.

I also drilled down on copper. The demand for smoother copper is increasing, which affects the industry standards. I'm involved in the IPC committee for copper and the committee for glass reinforcements. I chair IPC-4101, the IPC committee for PCB laminates. I'm engaged in the industry consortia with OEMs, fabricators, and raw material suppliers.

Nolan Johnson: What's really happening in the industry around those groups? 

Hitchcock: In my SMTA presentation, I talked about the North American market, how the book-to-bill has been doing lately, and then I shifted quickly into talking about density.

At a high level, packaging density drives both PCB density and substrate density. As your packages get more stuff packed in—10 pounds of stuff in a five-pound bag, for example—everything gets smaller and more compact, and there's more I/O. That drives the packaging technology, which in turn, drives the PCB technology.

Johnson: Which then drives the materials?

Hitchcock: Yes. From a materials perspective, they're getting thinner. The electrical drivers are making for a lower dielectric constant, but that's not the primary driver. The primary driver is the desire to get lower dissipation factor, or loss tangent. The next driver is to get the copper smoother. 

The copper challenge is adhesion; as your resin systems become lower and lower loss, it's difficult to get them to stick to anything. At the same time, copper is getting smoother, which makes it harder to adhere to resin systems. Those are some of the emerging challenges for both copper and the resin system. As you continue to add filler, you have increasingly lower percentages of the resin in that system, which challenges adhesion. Glass transition temperatures in newer materials are getting higher, which makes them more brittle. The fillers make them brittle, too.

At the same time, the packaging is adding I/O, and the sizes of the packages are getting larger, especially in infrastructure, but also in the mobile space. The XY dimensions are growing to accommodate the I/O, plus the pad sizes are getting smaller. This means the surface area with the volume of your solder—the surface area to your BGA pads, both on the package side and on the PCB side—are getting smaller. Meanwhile the adhesion is getting more challenging and the materials more rigid. Suddenly, CTE mismatch becomes a much bigger challenge.

Johnson: And you might be talking about ball grid arrays for most of the packages that have thousands of potential solder failures per package.

Hitchcock: Yes, the array packages and their increasing density is a challenge on materials. There are some creative things to be done on the material side to either absorb the strain between the package and the PCB, or to make them more robust to the stresses that they encounter.

Happy Holden: Electric vehicles are challenging. They're using the latest chip technology, but they want subassemblies which are low-cost but high reliability for a longer life. There are the mobile phone requirements for small form factors and power efficiency; that's a real dilemma.

Hitchcock: That's where substrate—like or near—substrate technology comes into play. It’s the space between a substrate or packaging; they’re circuit boards but located on the packaging side vs. the traditional PCB side. The line is blurring, and within that blurring is the desire for the density of a substrate but at the cost of a PCB.

Continue reading the rest of this conversation in the September 2023 issue of PCB007 Magazine.

Reference

1. ‘Advanced Technologies for Printed Circuit Board Fabrication and Raw Materials’, SMTA Oregon Expo 2023 keynote presentation.